SQU Journal for Science, 2014, 19(1), 8-14
© 2014 Sultan Qaboos University
Synthesis and Spectroscopic Properties
of a Fluorosensor for Zn2+ ions
Nawal K. Al-Rasbi*, Bushra Al-Wihibi and Nada Al-Nofali
Department of Chemistry, College of Science, Sultan Qaboos University, P.O. Box: 36, PC
123, Al-Khod, Muscat, Sultanate of Oman. *Email: [email protected].
ABSTRACT: The new Schiff base ligand L: (E)-N'-(pyridin-2-ylmethylene)acetohydrazide, was synthesized,
and its reaction with Zn(II) ions form the complex: [ZnL2](ClO4)2, as confirmed by X-ray crystallography.
This complex is stable in polar and non-polar solvents as proven by NMR. A significant enhancement in the
fluorescence was observed from L upon coordination to Zn(II) ions over other transition metals such as
Fe(II), Co(II), Ni(II), Cu(II), Cd(II) and Ag(I). These results suggest that L can be used as a selective
flourosensor for the detection of Zn(II) ions.
Keywords: Schiff base; Zinc(II); Crystal structure; Flourosensor.
‫التركيب والخصائص الضوئية لحساس طيفي أليونات الزنك‬
‫ بشرى الوهيبي و ندى النوفلي‬،‫نوال الراسبي‬
‫ َنقذ دل‬.‫[ كما ثبج مه انخزكيب انبهُري‬ZnL2](ClO4)2 :‫ َدراست حفاعهً مع أيُواث انشوك نخكُيه انمزكب‬L ‫ حم حزكيب شيفهيجىذ‬:‫ملخص‬
‫ ٌذا َقذ الحظىا سيادة في انطيف انضُئي نهماوح في‬.‫انزويه انمغىاطيسي انٍيذرَجيىي بأن انمزكب ثابج في انمحانيم انقطبيت َغيز انقطبيت‬
‫ كحساص ضُئي‬L ‫ ٌذي انىخائج حزشح انهيجىذ‬.‫ انىحاص َانفضت‬،‫ انىيكم‬،‫ انكُبانج‬،‫مزكب انشوك مقاروت بمزكباث فهشيت أخزِ حخكُن مه انحذيذ‬
.‫مخخصص السخكشاف أيُواث انشوك‬
.‫ انطيف انضُئي‬،‫ حزكيب بهُري‬،)II(‫ سوك‬،‫ شيفهيجىذ‬:‫مفتاح الكلمات‬
1. Introduction
I
n recent years, there was a great interest in Schiff base ligands for their unusual photophysical properties.
Their compounds have potential significance in light emitting diodes [1], as models in bioinorganic
chemistry [2] and as chemical sensors [3]. The Schiff base ligands with polydentate donors such as imine are
known for their selective detection of metal ions and other anions [4]. Many research groups have designed
extensive Schiff base derivatives by incorporating fluorescent units. Sensing of transition metal ions has
become an important goal in chemistry because of their important biological and environmental roles [5].
Zinc is an essential co-factor in many biological processes like brain function and pathology [6]. The main
challenge in the design of an effective fluorescent probe for Zn 2+ is to create a probe that responds selectively
to Zn2+ over other metal ions such as Ca2+, Mg2+ and Cd2+ [7]. A variety of Zn 2+ sensors have been reported
based on quinoline, [8] coumarin [9] and Schiff base ligands [7].
8
SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF A FLUOROSENSOR FOR Zn2+ IONS
In this paper, we describe the synthesis and spectroscopic characterization of a tridentate Schiff base
ligand L (Scheme 1). The ligand was designed by incorporating pyridyl and acetyl groups that have a strong
affinity for metal ions. The ligand is poorly fluorescent due to excited state electron transfer in the C=N bond.
Upon complexation with metal ions, the structure’s rigidity inhibits the excited-state electron transfer process,
and hence the fluorescence is produced. Our work also includes an investigation of the selectivity of L to Zn2+
ions by exploring the fluorescence properties in solution.
H
N
H
N
But
Me
Ph
N
N
N
O
O
OH
tBu
L
SL
Scheme 1. A presentation of the Schiff base ligand L used in this work.
2.
Results and discussion
2.1 Synthesis and crystal structure studies
The Schiff base ligand used, L, was generated by the condensation reaction of
stoichiometric,,amounts,,of, acetic hydrazide ,with picolinaldehyde according to the standard method used for
the synthesis of Schiff base ligands [10]. All of the spectroscopic and analytical data were consistent with the
correct formulation of the ligand. The IR spectrum of L showed a strong imine absorption, vC=N at 1627 cm–1.
The reaction of L with Zn(ClO4)2 in 1:2 molar ratio afforded colorless crystals with the general empirical
formula [ZnL2](ClO4)2 by elemental analysis (Scheme 2). The X-ray structure showed the simple
mononuclear Zn(II) complex: [ZnL2](ClO4)2∙MeOH (ZnL) that is presented in Figure 1 and Table 1. Initially,
we thought that ZnL may adapt a dimeric structure similar to other related Schiff base systems such as
[Zn2SL2(CH3CO2)(C2H6OH)] reported by Peng et al.[11] because the acetyl oxygen atoms are not basic
enough to coordinate to two metal centers.
O
H
N
H
N
H2N
+
H
N
Me
EtOH
O
N
O
H
N
N
+
Zn(NO3)2.2H2O
+
L
Me
N
2L
Me
N
Zn
MeOH
O
O
N
Me
N
H
N
ZnL
Scheme 2. Schematic presentation of the synthetic routes for L and its Zn(II) complex ZnL.
9
H2O
NAWAL K. AL-RASBI ET AL
However, the Zn(II) ion was found to be in a 6-coordinate N4O2 environment, coordinating to two
ligands in a meridional fashion. The bond distances to the Zn(II) ion were comparable to related Zn(II)
compounds. The Zn–N bonds to the imine donor (2.073(4) Å) were slightly shorter than to the pyridyl donor
(2.153(4) Å). The Zn–Oac bond distances were the longest, 2.164(3) and 2.222(3) Å. The angle between the
two Zn(NNO)2 planes was 92.78°. The crystal packing for ZnL exhibited intermolecular interactions
between the alternating pyridine rings forming parallel chains of alternating molecules. The remaining
perchlorate ions and methanol molecules exhibited a combination of O···HC and NH···O hydrogen contacts
within the molecules (Figure 1 (b)). These have very important consequences on the emission properties of
the complex.
(a)
(b)
Figure 1. (a) View of the complex cation of ZnL. The counter ions and solvent molecules are omitted for clarity. (b) The
crystal packing of ZnL, showing the packing of molecules into parallel chains by hydrogen contacts of O···HC in the
range 2.448–3.090 Å and NH···O in the range of 2.482–2.527 Å.
Table 2. Crystallographic data for ZnL.
Table 1. Bond lengths [Å] and angles [] for ZnL.
Zn(1)-N(17)
Zn(1)-N(7)
Zn(1)-N(1)
Zn(1)-N(12)
Zn(1)-O(21)
Zn(1)-O(11)
2.071(4)
2.077(4)
2.148(4)
2.158(4)
2.164(3)
2.222(3)
N(17)-Zn(1)-N(7)
N(17)-Zn(1)-N(1)
N(7)-Zn(1)-N(1)
N(17)-Zn(1)-N(12)
N(7)-Zn(1)-N(12)
N(1)-Zn(1)-N(12)
O(21)-Zn(1)-O(11)
N(7)-Zn(1)-O(21)
N(17)-Zn(1)-O(11)
N(1)-Zn(1)-O(21)
N(12)-Zn(1)-O(21)
165.89(15)
118.00(15)
76.05(15)
76.35(15)
99.95(14)
103.06(14)
92.80(12)
108.85(14)
92.79(13)
88.63(13)
150.82(14)
Formula
Formula weight
T (K)
Crystal system
a (Å)
b (Å)
c (Å)
 (°)
 (°)
 (°)
V(Å3), Z
Dcalc/ (mm-1)
Crystal size (mm)
Reflections collected
Data / restraints/ parameters
Goodness-of-fit on F2
Final R indices
Largest diff. peak and hole
(e.Å-3)
10
C17H22N6Cl2O11Zn
622.68
100(2) K
Triclinic, P–1
9.6126(11)
11.8491(13)
12.7991(15)
112.255(4)
111.853(4)
96.455(4)
1196.5(2), 2
1.728/ 1.321
0.16 x 0.15 x 0.15
15386
5857 / 0 / 283
1.050
R1 = 0.0725
wR2 = 0.1886
2.012 and -1.526
SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF A FLUOROSENSOR FOR Zn2+ IONS
2.2
1
H NMR studies
The 1H NMR spectra of re-dissolved crystals of Zn(II) complex ZnL, were investigated in CDCl3 and
DMSO-d6 (Figure 2). The spectra exhibited the number of peaks expected for the molecular structure and
confirmed the stability of ZnL in solution. In general, it was found that the complex ZnL exhibits a 2-fold
symmetry, such that the two coordinating ligands are equivalent to one another. It was noticed in the 1H NMR
spectrum of ZnL in DMSO-d6, that the N–H and CH3 signals are strongly downfield shifted to 11.50 and 3.40
ppm indicating that these protons are under a rich electronic zone via H-bonding with DMSO molecules. The
pyridine and CH=N protons are slightly down-field shifted when compared to those in CDCl3. These results
suggest that the pyridine and CH=N protons of ZnL are under the shielding zone of -electrons of a
conjugated system due to aggregation of molecules in non-polar solvents via – stacking interactions as
referred from the crystal packing [12].
Figure 2. The 1HNMR spectra of re-dissolved crystals of ZnL a) in CDCl3 and b) DMSO-d6 at ambient temperature.
2.3
Luminescence properties
The UV/Vis absorption spectra of the ligand and its Zn(II) complex are shown in Figure 3a. The ligand
exhibited strong absorption in the UV region at 304 nm ( ~38,100 M–1 cm–1) and a shoulder at ca. 318 nm.
These transitions could be attributed to –* transitions of the aromatic rings and the imine bonds present in
the molecules [13]. Upon complexation to Zn(II) ion, a distinct red shift of the high energetic band was
observed at 313 nm ( ~47,500 M–1 cm–1). The band was highly intense and unresolved. However, a new
weak band observed at 363 nm ( ~13,000 M–1 cm–1) contributed to the formation of the lowest-energy
excited state of ligand centered –* transitions in L–Zn(II) bonds [14].
The ligand, L has no emissive chromophores and hence, is not luminescent. Excitation of the complex
ZnL in DCM at 363 nm, exhibited an enhanced emission centered at 468 nm (Figure 3b). Excitation at the
highest energy absorption band, 313 nm, revealed very weak emission. Normally in the presence of
coordinating Zn(II) ions, the fluorescence of a ligand is enhanced by preventing the photo induced electron
transfer (PET) process in the ligand [15]. This explains the intense emission of the complex at an excitation
wavelength of 368 nm as this absorption is based on L–Zn(II) transition.
In switching to more polar solvents such as THF, the effect of polarity is much more pronounced in the
emission of ZnL (Figure 3b). The emission spectrum showed an intense band with a large red-shift to 480 nm
with respect to those in DCM. The 1H NMR results suggest that ZnL molecules can form aggregates in a less
polar DCM solution. In switching to a polar THF solution, the molecules of ZnL are under the influence of
11
NAWAL K. AL-RASBI ET AL
Fluorescence intensity
H-bonding with THF molecules. This would cause random aggregates of ZnL in THF, and hence the
emission energy of ZnL is red shifted [16-17].
(b)
(a) 0.5
400
Absorbance
0.4
0.3
0.2
0.1
300
200
100
0
0
280
330
380
270
430
370
470
570
Wavelength/nm
Wavelength/nm
Figure 3. (a) The absorption spectra of L (dashed line) and ZnL (solid line) in DCM at RT. (b) Emission (solid line) and
excitation (dashed line) spectra of ZnL in 1.0 x 10–5 M DCM (black) and THF (red) solutions at RT.
The selectivity of L towards Zn(II) ions was also investigated by the titration of the solution of L in
DCM with a methanoic solution of Zn(ClO4)2 (0–1.5 equiv.). The bands of these spectra showed systematic
enhancement in fluorescence which was very similar to that of ZnL complex after the addition of few
equivalents of Zn(II) ions. The maximum fluorescence intensity was observed at a molar stoichiometry of 1:2
between Zn(II) ions and L. This is the point where all L molecules coordinate to Zn(II) ions to form the
complex ZnL (1:2 molar ratio). Further addition of Zn(II) ions revealed no enhanced emission. However, in
switching to a polar THF solvent, addition of Zn(II) to L enhanced the emission in a similar manner to the
complex ZnL, as the emission was red-shifted.
The emission behavior of L upon titration with other d-metals such as Fe(II), Co(II), Ni(II) and Cu(II)
was also investigated. The emission of their complexes did not enhance the emission of L. These metals
exhibited electronic d-d transitions that caused the quenching of emission in their complexes. For Zn(II), the
metal has a fully filled d-shell, d10 and exhibits no such d-d transitions. Instead, coordination to a Zn(II) center
would enhance emission from the coordinating ligand. However, the emission of L was also studied with
other d10 metals such as Cd(II) and Ag(I). These metals failed to induce the fluorescence from L, probably
because these metals failed to form a suitable coordination environment around L to inhibit the PET process
for their larger ionic radii. The high selectivity of L towards Zn(II) ions over other metal ions suggests that
the ligand L can act as a fluorosensor for Zn(II).
3.
Conclusions
The synthesis and crystal structure of the Zn(II) complex with a new tridentate Schiff base ligand 'L' has
been investigated and fully characterized. The structure of the complex: [ZnL2](ClO4)2 is retained in polar and
non-polar solvents as confirmed by NMR. It has been found that the emission of L is induced upon
coordination to Zn(II) over other metal ions such as Fe(II), Co(II), Ni(II), Cu(II), Cd(II) and Ag(I). These
results suggest that L can be used as a selective fluorsensor for the detection of Zn(II) ions.
4.
Experimental
4.1 General details
All organic reagents, metal salts and solvents were purchased from Sigma-Aldrich and were used as
received without further purification. Infrared spectra were recorded as KBr pellets on a Perkin Elmer FT-IR
spectrometer BX in the range of 4000-400 cm–1. Electrospray (ES) mass spectra were recorded on a VG
Autospec magnetic sector instrument. Proton magnetic resonance (1H-NMR) spectra were recorded using a
500 MHz AVANCE Bruker NMR spectrometer. Chemical shifts (δppm) were recorded in parts per million
12
SYNTHESIS AND SPECTROSCOPIC PROPERTIES OF A FLUOROSENSOR FOR Zn2+ IONS
(ppm) downfield from TMS (assigned as zero ppm). Elemental analyses for carbon, hydrogen and nitrogen
were performed using a Perkin Elmer 2400 CHNS/ O Series II Elemental Analyser.
4.2 Preparation of ligand
The ligand L was prepared according to the general method for preparation of Schiff bases. A solution
of pyridine-2-aldehyde (1.1 g; 10 mmol) in EtOH (20 mL) was added dropwise to a solution of acetic
hydrazide (0.65 g; 10 mmol) in EtOH (20 mL) and refluxed at 85 C for four hours. The reaction mixture was
concentrated under vacuum and cooled in the fridge to yield 1.40 g (86 %) of L. ES-MS: m/z= 164.0 [M+]
186.0 [Na+M+]. IR (KBr, cm-1) 3450, 3010, 2944, 1627, 1394-1600. 1H NMR (CDCl3): 10.20 (1H, s, NH),
8.61 (1H, d, CH), 8.05 (1H, s, py H6), 7.93 (1H, d, py H3), 7.73 (1H, t, py H5), 7.28 (1H, t, py H4) and 2.40
(3H, s, Me). C8H9N3O (163.01): calcd. C 58.88, H 5.56, N 25.75; found C 58.94, H 5.62, N 25.69%.
4.3 Preparation of [ZnL2](ClO4)2MeOH, ZnL
To a solution of L (0.27 g; 1.66 mmol) in MeOH (10 mL) was added dropwise a solution of Zn(ClO4)2
(0.22 g; 0.83 mmol) dissolved in MeOH (5 mL) at room temperature. The yellowish solution which formed
was left for the solvent to slowly evaporate off. Colorless needles formed within few days. Yield 88%. ESMS: m/z 393.77. IR (KBr, cm-1) 1623.1H NMR (DMSO-d6): 11.50 (1H, s, NH), 8.58 (1H, s, CH), 8.14 (1H, d,
J= 9 Hz py H6), 7.80-7.93 (2H, m, py H3,H5), 7.35 (1H, dt, py H4) and 3.28 (3H, s, Me). ZnC16H18N6O10Cl2
(590.64): calcd. C 32.54, H 3.07, N 14.23; found C 32.63, H 3.11, N 14.21%.
4.4 Spectrofluorimetric measurements
UV-visible spectra were recorded on a Varian Cary 50 conc UV-visible spectrophotometer in the range
250-800 nm. Quartz cuvettes of 1 cm path length were used and solvent background corrections were applied.
Steady state luminescence spectra were recorded on a Perkin-Elmer LS50B fluorimeter. Solution spectra were
measured using 1 cm quartz cuvettes. The spectrofluorimetric titrations were performed as follows. Stock
solutions of the ligands (ca. 1.0 x 10−5 M) were prepared by dissolving an appropriate amount of the ligand in
a 50 mL volumetric flask and diluting it to the mark with DCM or THF. Titrations were carried out by the
addition of µL amounts of standard solutions of Zn(ClO4)2, Fe(ClO4)2, Co(ClO4)2, Ni(ClO4)2, Cu(ClO4)2,
Cd(SO4)2 and AgBF4 in MeOH to a solution of L in DCM or THF.
4.5
Crystallography
Single crystals of the complex were obtained as detailed above. The crystal structure of ZnL,
[ZnL2](ClO4)2∙MeOH, was carried out at the University of Sheffield. Data was collected at 100 K, using a
Bruker APEX-II CCD diffractometer equipped with Mo-Kα radiation. Absorption corrections were applied in
each case using SADABS [18]. The structures were solved by direct methods and refined by full matrix least
squares methods on F2 using SHELXL-97 [19,20]. The hydrogen atoms were generated geometrically with
isotropic thermal parameters. Some oxygen atoms in perchlorates were disordered and refined with isotropic
displacement parameters. The MeOH molecule was refined with isotropic displacement parameters to keep
the refinement stable.
Crystallography data (excluding the structure factors) for the reported structure have been deposited
with the Cambridge Crystallographic Data Centre CCDC 943597.
5.
Acknowledgements
We would like to thank Mrs. Susan Bradshaw (University of Sheffield, UK) for recording the 1HNMR
spectra of the compounds. We also thank Sultan Qaboos University for financial support.
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Received 10 February 2014
Accepted 3 April 2014
14